Lubricating oil compositions with improved oxidative performance comprising alkylated diphenylamine antioxidant and sulfonate detergents

ABSTRACT

Lubricating oil composition is provided. The composition includes several components including a base oil, a primary antioxidant including alkylated diphenylamines having an alkyl group derived from a propylene tetramer; and a sulfonate detergent.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Provisional Application entitled“IMPROVED OXIDATIVE PERFORMANCE WITH CARBOXYLATE DETERGENTS” (ATTORNEYDOCKET: T-11173) filed on Mar. 11, 2020, the contents of which areherein incorporated by reference.

TECHNICAL FIELD

This disclosure relates to lubricating oil additives that disruptoxidation and increase the useful life of lubricating oils. Moreparticularly, this disclosure relates to lubricating oil compositionsthat include alkylated diphenylamine antioxidant and sulfonatedetergent.

BACKGROUND

Oxidation is a concern for in-service lubricating oils as it can causethickening of the oil, sludge, varnish, acid number increase andcorrosion. These outcomes are generally detrimental to proper operationof automotive engines and limit useful life of the lubricating oil. Withcontinually evolving engine designs, operating conditions and oilperformance expectations, oxidation continues to be an important ongoingtechnical challenge.

One way to slow down oxidation in engines is to introduce antioxidantsto lubricating oils. Additionally, antioxidants can also extend drainintervals, maintain viscosity, reduce deposit, reduce foam formation,protect against corrosion as well as protect the lubricating oil againsthigh temperature.

There are many antioxidants that have varying degrees of effectiveness.Commercial lubricants are usually formulated with one or moreantioxidants to protect the fluid under a wide range of conditions(e.g., temperature, time, air mixtures, pressure, etc.).

In particular, alkylated diphenylamines are used as antioxidants.Widely-used alkylated diphenylamine antioxidants include nonylated (C9)diphenylamine which can be added into organic fluids such as engineoils, gear oils, hydraulic fluids, compressor oils, turbine oils, andgrease.

SUMMARY

This disclosure relates to lubricating oil additives that disruptoxidation and increase the useful life of lubricating oils. Moreparticularly, this disclosure relates to compositions that includealkylated diphenylamines and sulfonate detergents.

In one aspect, there is provided a lubricating oil compositioncomprising: a base oil; a primary antioxidant comprising alkylateddiphenylamines having an alkyl group derived from propylene tetramers;and a sulfonate detergent.

In a further aspect, there is provided a method of improving oxidationstability of a lubricating oil, the method comprising: supplying to anengine a lubricating oil composition comprising: a base oil; a primaryantioxidant comprising alkylated diphenylamines having an alkyl groupderived from propylene tetramers; and a sulfonate detergent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a comparison of oxidative induction time offormulated oil samples as described in the Example.

DETAILED DESCRIPTION

In this specification, the following words and expressions, if and whenused, have the meanings ascribed below.

The term “antioxidant” or equivalent term (e.g., “oxidation stabilizer”or “oxidation inhibitor”) refers to a composition and its ability toresist deleterious attacks in an oxidizing environment. Antioxidants areoften used in organic fluids (e.g., lubricating oil, gear oil,compressor oil, mineral oil, hydraulic fluid, etc.) to improve theoxidation stability of the organic fluid.

The term “alkyl” or related term refers to a saturated hydrocarbongroup, that can be linear, branched, cyclic, or a combination of cyclic,linear and/or branched.

The term “olefin” refers to a hydrocarbon that has at least onecarbon-carbon double bond that is not part of an aromatic ring or ringsystem. Olefins may include aliphatic and aromatic, cyclic and acyclic,and/or linear and branched compounds having at least one carbon-carbondouble bond that is not part of an aromatic ring or ring system, unlessspecifically stated otherwise. Olefins having only one, only two, onlythree, etc., carbon-carbon double bonds can be identified by use of theterm “mono,” “di,” “tri,” etc., within the name of the olefin. Theolefins can be further identified by the position of the carbon-carbondouble bond(s). Depending on the context, the term “olefin” may refer toan “olefin oligomer” or to an “olefin monomer” or both.

An “olefin oligomer” is an oligomer made from oligomerization of “olefinmonomers.” For example, a “propylene oligomer” is made from theoligomerization of propylene monomers. Examples of propylene oligomersinclude propylene tetramer and propylene pentamer. A “propylenetetramer” is an olefin oligomer product resulting from theoligomerization of nominally 4 propylene monomers. These terms also canbe used generically to describe homo-oligomers, co-oligomers, salts ofoligomers, derivatives of oligomers, and the like.

A “minor amount” or related term means less than 50 wt % of acomposition, expressed in respect of the stated additive and in respectof the total weight of the composition, reckoned as active ingredient ofthe additive.

A “major amount” or related term means an amount greater than 50 wt %based on the total weight of the composition.

Antioxidant Composition

The present invention relates to antioxidant compositions that disruptoxidation and increase the useful life of lubricating oils. Moreparticularly, the present invention describes antioxidant compositionscomprising a plurality of lubricant additives. The lubricant additivesinclude at least one antioxidant and at least one detergent workingtogether to provide enhanced oxidative performance. The enhancedperformance is a result of a previously unknown synergy arising from thelubricant additive components of the present invention in lubricatingoil compositions. Antioxidants and detergents compatible with thepresent invention will be described herein.

Primary Antioxidant

The antioxidant composition comprises a primary antioxidant and one ormore secondary antioxidants. The primary antioxidant of the presentinvention is an alkylated diphenylamine having one or more relativelylong alkyl groups. Conventional alkylated diphenylamine antioxidantstypically utilize relatively short alkyl groups. These include, forexample, nonylated diphenylamine (“propylene trimer”) which nominallyhas 9 carbons and can be formed from the oligomerization of propylene.

The alkylated diphenylamines of the present invention have beenalkylated by propylene tetramers (having nominally 12 carbons) or by amixture comprising propylene tetramers, wherein the propylene tetrameris the predominant olefin oligomer alkylating agent. Propylene tetramerscan be obtained by the oligomerization of 4 propylene monomers. Thereare several potential advantages of propylene tetramer over propylenetrimer including, but not limited to, increased oil solubility, cheapercost, and superior stability against oxidation.

The alkylated diphenylamine of the present invention may be present atabout 0.4 wt % to about 20 wt % of the lubricating oil composition, suchas from about 0.5 wt % to about 15 wt %, 0.1 wt % to about 10 wt %, 0.5wt % to about 8 wt %, or 1 wt % to about 5 wt %.

Propylene Oligomers

The propylene oligomers (i.e., propylene tetramers) of the presentinvention can be prepared by any compatible method known in the art. Byway of an example, a process for preparing the propylene oligomersemploys a liquid phosphoric acid oligomerization catalyst. Descriptionsof liquid phosphoric acid-catalyzed propylene oligomerization processcan be found in U.S. Pat. Nos. 2,592,428; 2,814,655; and 3,887,634, therelevant portions of which are hereby incorporated by reference.

An unrefined product of oligomerization process typically includes amixture of branched olefins having a distribution in number of carbons.In a commercial setting, olefin oligomers are subject to extremeconditions during the oligomerization process which results in cracking,recombination, isomerization and the like. Refined or processedoligomerization products typically have higher concentration of thedesired product. Thus, the term “propylene tetramer” may not necessarilyrefer to a pure propylene tetramer product but a mixture of olefins orolefin oligomer products. Accordingly, the product of alkylationinvolving diphenylamine and propylene tetramer can have a distributionin number of carbons within the alkylated alkyl groups.

Propylene tetramers can be obtained from the oligomerization of 4propylene monomers. The propylene tetramer is a cost effective olefin tomanufacture. As a product of oligomerization, it features a highlybranched chain of 10 to 15 carbons with high degree of methyl branchingthat imparts exceptional oil solubility and compatibility with other oilsoluble lubricant additive components. In some embodiments, the averagecarbon number can range from about 10 to about 15.

The product of oligomerization can vary in degree of branching. Forexample, the propylene tetramer can exhibit a total branching (i.e., sumof olefinic and aliphatic branching) ranging from 1 to 15. In someembodiments, the average total branching can range from about 1 to about15.

The propylene tetramers of the present invention generally comprise atleast 50 wt % of C₁₀ to C₁₅ carbon atoms. In an embodiment, thepropylene tetramers contain a distribution of carbon atoms whichcomprise at least 60 wt % of C₁₀ to C₁₅ carbon atoms. In an embodiment,the propylene tetramers contain a distribution of carbon atoms whichcomprise at least 70 wt % of C₁₀ to C₁₅ carbon atoms. In an embodiment,the propylene oligomers contain a distribution of carbon atoms whichcomprise at least 80 wt % of C₁₀ to C₁₅ carbon atoms. In an embodiment,the propylene oligomers contain a distribution of carbon atoms whichcomprise at least 90 wt % of C₁₀ to C₁₅ carbon atoms.

As will be apparent to a person of ordinary skill in the art, thepropylene oligomers employed herein may also contain a minor amount oflower molecular weight propylene oligomer(s) such as propylene trimer,as well as higher molecular weight propylene oligomer(s) such aspropylene pentamer. For example, the propylene tetramer of the presentinvention may be a mixture of olefinic hydrocarbons containing 0-1 wt %C₉H₁₈, 0-5 wt % C₁₀H₂₀, 0-10 wt % C₁₁H₂₂, 50-90 wt % C₁₂H₂₄, 10-20 wt %C₁₃H₂₆, 5-15 wt % C₁₄H₂₈, and/or 1-10 wt % C₁₅H₃₀.

Alkylation

The alkylated diphenylamine of the present invention can be obtained byany alkylation process compatible with the present invention. Forexample, US 6,355,839, hereby incorporated by reference, describes thepreparation of alkylated diphenylamine wherein the diphenylamine isalkylated with polyisobutylene.

Any suitable catalyst may be used. For example, the alkylation ofdiphenylamine may proceed in the presence of a clay catalyst.Temperature of this reaction can range from 140° C. to 200° C., moretypically between 150° C. to 190° C. In some embodiments, thetemperature of the reaction ranges between 160° C. to 180° C. Thereaction can be carried out at a single temperature, or sequentially, atdifferent temperatures. The propylene oligomer can be charged at acharge mole ratio (CMR) between 2:1 to 8:1 in relation to thediphenylamine charge. In some embodiments, the CMR is between 3:1 to 7:1or between 4:1 and 6:1. The reaction product can be filtered to removethe catalyst and then distilled to remove unreacted olefin oligomers anddiphenylamines. The use of clay as catalyst is disclosed in U.S. Pat.No. 3,452,056, which is hereby incorporated by reference.

As would be expected to a person of ordinary skill in the art, thereaction conditions may vary significantly depending on the catalystused. For example, reactions involving homogeneous acid catalysts mayonly require temperatures ranging between 75° C. to 100° C.

Depending on the reaction conditions, the alkylated diphenylamineproduct can have various relative amounts of mono-alkylated,di-alkylated, and/or tri-alkylated diphenylamine products. It should beapparent that for a given di- or tri-alkylated diphenylamine molecule,the two or more alkylated alkyl groups may be identical or different inaccordance with this disclosure.

Secondary Antioxidants

The present invention employs one or more secondary antioxidants incombination with the primary antioxidant. The secondary antioxidant maybe present at about 0.01 wt % to about 20 wt % of the lubricating oilcomposition, such as from about 0.05 wt % to about 15 wt %, 0.1 wt % toabout 10 wt %, 0.5 wt % to about 8 wt %, or 1 wt % to about 5 wt %.

A number of secondary antioxidants are compatible with the presentinvention. Examples of secondary antioxidants include molybdenumsuccinimides, dithiocarbamates and hindered phenols. These oil-solublecomponents are generally known.

For example, the mono and polysuccinimides that can be used to preparethe molybdenum complexes described herein are disclosed in numerousreferences and are well known in the art. Certain fundamental types ofsuccinimides and the related materials encompassed by the term of art“succinimide” are taught in U.S. Pat. Nos. 3,219,666; 3,172,892; and3,272,746, the disclosures of which are hereby incorporated byreference. The term “succinimide” is understood in the art to includemany of the amide, imide, and amidine species which may also be formed.The predominant product however is a succinimide and this term has beengenerally accepted as meaning the product of a reaction of an alkenylsubstituted succinic acid or anhydride with a nitrogen-containingcompound.

Preferred succinimides, because of their commercial availability, arethose succinimides prepared from a hydrocarbyl succinic anhydride,wherein the hydrocarbyl group contains from about 24 to about 350 carbonatoms, and an ethylene amine, said ethylene amines being especiallycharacterized by ethylene diamine, diethylene triamine, triethylenetetramine, and tetraethylene pentamine. Particularly preferred are thosesuccinimides prepared from polyisobutenyl succinic anhydride of 70 to128 carbon atoms and tetraethylene pentamine or triethylene tetramine ormixtures thereof.

Also included within the term “succinimide” are the cooligomers of ahydrocarbyl succinic acid or anhydride and a poly secondary aminecontaining at least one tertiary amino nitrogen in addition to two ormore secondary amino groups. Ordinarily this composition has between1,500 and 50,000 average molecular weight. A typical compound would bethat prepared by reacting polyisobutenyl succinic anhydride and ethylenedipiperazine.

Succinimides having an average molecular weight of 1000 or 1300 or 2300and mixtures thereof are most preferred. Such succinimides can be posttreated with boron or ethylene carbonate as known in the art.

Suitable dithiocarbamates include, but are not limited to,dithiocarbamates wherein the metal is zinc, copper or molybdenum,ashless thiocarbamates or dithiocarbamates (i.e., essentially metalfree) such as methylenebis(dialkyldithiocarbamate),ethylenebis(dialkyldithiocarbamate), and isobutyldisulfide-2,2′-bis(dialkyldithiocarbamate) where the alkyl groups of thedialkyldithiocarbamate can preferably have from 1 to 6 carbon atoms.Examples of preferred ashless dithiocarbamates aremethylenebis(dibutyldithiocarbamate), ethylenebis(dibutylthiocarbamate)and isobutyl disulfide-2,2′-bis(dibutyldithiocarbamate).

The secondary antioxidant employed in the lubricating oil of the presentinvention may be a sterically hindered phenol. The hindered phenolantioxidant often contains a secondary butyl and/or a tertiary butylgroup as a sterically hindering group. The phenol group is often furthersubstituted with a hydrocarbyl group and/or a bridging group linking toa second aromatic group. Suitable hindered phenols include, but are notlimited to, 2,6-di-tert-butylphenol, 4-methyl-2,6-di-tert-butylphenol,4-ethyl-2,6-di-tert-butylphenol, 4-propyl-2,6-di-tert-butylphenol or4-butyl-2,6-di-tert-butylphenol, or 4-dodecyl-2,6-di-tert-butylphenol.

Detergents

The antioxidant composition of the present invention includes one ormore detergents. Detergents may be present at about 0.01 wt % to about10 wt % of the lubricating oil composition, such as from about 0.05 wt %to about 8 wt %, 0.1 wt % to about 5 wt %, 0.5 wt % to about 4 wt %, or1 wt % to about 3 wt %.

Detergents are normally salts (e.g., overbased salts) and are singlephase, homogeneous Newtonian systems characterized by a metal content inexcess of that which would be present according to the stoichiometry ofthe metal and the particular acidic organic compound reacted with themetal.

The detergents of the present invention include sulfonate detergents.Metallic detergents such as sulfonate detergents typically contain apolar head group and a hydrocarbon tail or an oleophilic group. Ingeneral, the hydrocarbon tail can range from about 3 carbons to 50carbons in length.

Sulfonate detergents may be natural or synthesized. The detergent may beneutral or overbased. Overbased detergents may range in degree ofoverbasing (as measured by ASTM D2896). Compatible overbased sulfonatesinclude low overbased, medium overbased, high overbased, and highoverbased sulfonate detergents. In some embodiments, the detergent maybe borated.

Examples of sulfonate detergents include alkyl aryl sulfonates and thelike. Specific examples include magnesium alkyltoluene sulfonates whichare described in US20110136711. Other examples include calcium alkylaryl sulfonates, calcium alkyltoluene sulfonates, and magnesiumalkylbenzene sulfonates.

Metals of detergents can also include alkali or alkaline earth metals,e.g., barium, sodium, potassium, lithium, calcium, and magnesium. Themost commonly used metals are calcium and magnesium, which both may bepresent in detergents used in lubricants, and mixtures of calcium and/ormagnesium with sodium.

In some embodiments, additional detergents may be used. The additionaldetergents include phenates, salicylates, phenolates, phosphonates,thiophosphonates, ionic surfactants, and the like. In some embodiments,additional detergents include hybrid and/or complex detergents.

Lubricating Oil Compositions

The antioxidant compositions of present disclosure may be used inlubricating oil to impart oxidation stability to the lubricating oil.The primary antioxidant, secondary antioxidant, and one or moredetergents may be present in any ratio provided that theirconcentrations fall within the guidelines provided herein.

In general, the antioxidant compositions are oil soluble meaning thatthey are, for instance, soluble or stably dispersible in oil to anextent sufficient to exert their intended effect in the environment inwhich the oil is employed. Moreover, the additional incorporation ofother additives may also permit incorporation of higher levels of aparticular additive, if desired. The term oil-soluble does notnecessarily indicate that the compounds or additives are soluble,dissolvable, miscible, or capable of being suspended in the oil in allproportions. If other antioxidants are present in the lubricating oilcomposition, a lesser amount of the antioxidant of the present inventionmay be used.

Oils used as the base oil will be selected or blended depending on thedesired end use and the additives in the finished oil to give thedesired grade of engine oil, e.g. a lubricating oil composition havingan Society of Automotive Engineers (SAE) Viscosity Grade of OW, OW-8,OW-16, OW-20, OW-30, OW-40, OW-50, OW-60, 5W, 5W-20, 5W-30, 5W-40,5W-50, 5W-60, 10W, 10W-20, 10W-30, 10W-40, 10W-50, 15W, 15W-20, 15W-30,or 15W-40. Straight grade based oils such as SAE 30, 40, 50, and 60 mayalso be used.

The oil of lubricating viscosity (sometimes referred to as “base stock”or “base oil”) is the primary liquid constituent of a lubricant, intowhich additives and possibly other oils are blended, for example toproduce a final lubricant (or lubricant composition). A base oil, whichis useful for making concentrates as well as for making lubricating oilcompositions therefrom, may be selected from natural (vegetable, animalor mineral) and synthetic lubricating oils and mixtures thereof.

Definitions for the base stocks and base oils in this disclosure are thesame as those found in American Petroleum Institute (API) Publication1509 Annex E (“API Base Oil Interchangeability Guidelines for PassengerCar Motor Oils and Diesel Engine Oils,” December 2016). Group I basestocks contain less than 90% saturates and/or greater than 0.03% sulfurand have a viscosity index greater than or equal to 80 and less than 120using the test methods specified in Table E-1. Group II base stockscontain greater than or equal to 90% saturates and less than or equal to0.03% sulfur and have a viscosity index greater than or equal to 80 andless than 120 using the test methods specified in Table E-1. Group IIIbase stocks contain greater than or equal to 90% saturates and less thanor equal to 0.03% sulfur and have a viscosity index greater than orequal to 120 using the test methods specified in Table E-1. Group IVbase stocks are polyalphaolefins (PAO). Group V base stocks include allother base stocks not included in Group I, II, III, or IV.

Natural oils include animal oils, vegetable oils (e.g., castor oil andlard oil), and mineral oils. Animal and vegetable oils possessingfavorable thermal oxidative stability can be used. Of the natural oils,mineral oils are preferred. Mineral oils vary widely as to their crudesource, for example, as to whether they are paraffinic, naphthenic, ormixed paraffinic-naphthenic. Oils derived from coal or shale are alsouseful. Natural oils vary also as to the method used for theirproduction and purification, for example, their distillation range andwhether they are straight run or cracked, hydrorefined, or solventextracted.

Synthetic oils include hydrocarbon oil. Hydrocarbon oils include oilssuch as polymerized and interpolymerized olefins (e.g., polybutylenes,polypropylenes, propylene isobutylene copolymers, ethylene-olefincopolymers, and ethylene-alphaolefin copolymers). Polyalphaolefin (PAO)oil base stocks are commonly used synthetic hydrocarbon oil. By way ofexample, PAOs derived from C₈ to C₁₄ olefins, e.g., C₈, C₁₀, C₁₂, C₁₄olefins or mixtures thereof, may be utilized.

Other useful fluids for use as base oils include non-conventional orunconventional base stocks that have been processed, preferablycatalytically, or synthesized to provide high performancecharacteristics.

Non-conventional or unconventional base stocks/base oils include one ormore of a mixture of base stock(s) derived from one or moreGas-to-Liquids (GTL) materials, as well as isomerate/isodewaxate basestock(s) derived from natural wax or waxy feeds, mineral and ornon-mineral oil waxy feed stocks such as slack waxes, natural waxes, andwaxy stocks such as gas oils, waxy fuels hydrocracker bottoms, waxyraffinate, hydrocrackate, thermal crackates, or other mineral, mineraloil, or even non-petroleum oil derived waxy materials such as waxymaterials received from coal liquefaction or shale oil, and mixtures ofsuch base stocks.

Base oils for use in the lubricating oil compositions of presentdisclosure are any of the variety of oils corresponding to API Group I,Group II, Group III, Group IV, and Group V oils, and mixtures thereof,preferably API Group II, Group III, Group IV, and Group V oils, andmixtures thereof, more preferably the Group III to Group V base oils dueto their exceptional volatility, stability, viscometric and cleanlinessfeatures.

Typically, the base oil will have a kinematic viscosity at 100° C. (ASTMD445) in a range of 2.5 to 20 mm²/s (e.g., 3 to 12 mm²/s, 4 to 10 mm²/s,or 4.5 to 8 mm²/s).

The present lubricating oil compositions may also contain conventionallubricant additives for imparting auxiliary functions to give a finishedlubricating oil composition in which these additives are dispersed ordissolved. For example, the lubricating oil compositions can be blendedwith antioxidants, ashless dispersants, anti-wear agents, detergentssuch as metal detergents, rust inhibitors, dehazing agents, demulsifyingagents, friction modifiers, metal deactivating agents, pour pointdepressants, viscosity modifiers, antifoaming agents, co-solvents,package compatibilizers, corrosion-inhibitors, dyes, extreme pressureagents and the like and mixtures thereof. A variety of the additives areknown and commercially available. These additives, or their analogouscompounds, can be employed for the preparation of the lubricating oilcompositions of the invention by the usual blending procedures.

Each of the foregoing additives, when used, is used at a functionallyeffective amount to impart the desired properties to the lubricant.Thus, for example, if an additive is an ashless dispersant, afunctionally effective amount of this ashless dispersant would be anamount sufficient to impart the desired dispersancy characteristics tothe lubricant. Generally, the concentration of each of these additives,when used, may range, unless otherwise specified, from about 0.001 toabout 20 wt %, such as about 0.01 to about 10 wt %.

The following illustrative examples are intended to be non-limiting.

EXAMPLES

As shown in FIG. 1 , oxidation induction times of fully formulatedengine oils were tested. The fully formulated engine oils include one ormore antioxidants and a sulfonate detergent as well as common lubricantadditives such as dispersant, and corrosion inhibitor.

The first engine oil samples (“DPA Only”) includes a sulfonate detergentand an alkylated diphenylamine. The alkylated diphenylamine is anonylated diphenylamine or a diphenylamine alkylated with a propylenetetramer. Gas chromatography analysis of the diphenylamine alkylatedwith propylene tetramer is summarized in Table 1 below. The analysisshows that roughly half of the sample is mono alkylated diphenylamine.Roughly another half of the sample is di alkylated diphenylamine. Thereis a very small amount of diphenylamine with C₃-C₈ alkyl group.

TABLE 1 DPA Dimer C3-C8 DPA Total Mono DPA Total Di DPA 0.0% 0.0% 0.6%50.8% 48.6%

Other engine oil samples include one or more additional antioxidants(i.e., molybdenum succinimide, hindered phenol, dithiocarbamate). In amixed engine oil sample with multiple antioxidants, each antioxidantpresent is present in equal treat levels/weight percent.

Test engine oil samples featuring two antioxidants include an alkylateddiphenylamine with molybdenum succinimide (“DPA/Mo succinimide”),hindered phenol (“DPA/hindered phenol”) or dithiocarbamate(“DPA/dithiocarbamate”). Test engine oil samples featuring threeantioxidants include an alkylated diphenylamine with molybdenumsuccinimide and hindered phenol (“DPA/Mo succinimide/hindered phenol”),molybdenum succinimide and dithiocarbamate (“DPA/Mosuccinimide/dithiocarbamate”), or hindered phenol and dithiocarbamate(“DPA/hindered phenol/dithiocarbamate”). Test engine oil samples featurefour antioxidants include an alkylated diphenylamine with molybdenumsuccinimide, hindered phenol, and dithiocarbamate (“DPA/Mosuccinimide/hindered phenol/dithiocarbamate”).

For each test sample, sulfonate detergent (low and medium overbasedsulfonates) is present in 78 mM while the total concentration of theantioxidant(s) is 1.5 wt %.

The data shows consistently higher oxidation induction times in sampleswith diphenylamine alkylated with propylene tetramers as compared tosamples with nonylated diphenylamine.

Oxidation induction times were evaluated using Pressurized DifferentialScanning calorimetry (PDSC) according to ASTM D6186 test protocol.Greater oxidation induction times indicated greater oxidation stability.

1. A lubricating oil composition comprising: a base oil; a primaryantioxidant comprising alkylated diphenylamines having an alkyl groupderived from a propylene tetramer; and a sulfonate detergent.
 2. Thelubricating oil composition of claim 1, further comprising: a secondaryantioxidant comprising a dithiocarbamate, a hindered phenol, or amolybdenum succinimide.
 3. The lubricating oil composition of claim 1,wherein at least 50% of the alkyl group of the alkylated diphenylaminehave a carbon number between 10 and
 15. 4. The lubricating oilcomposition of claim 1, wherein the sulfonate detergent is apetroleum-based detergent or a synthetic detergent.
 5. The lubricatingoil composition of claim 1, wherein the primary antioxidant is presentin 0.01 wt % to 20 wt % of the lubricating oil composition.
 6. Thelubricating oil composition of claim 1, wherein the sulfonate detergentis overbased.
 7. The lubricating oil composition of claim 1, wherein thesecondary antioxidant is present in 0.01 wt % to 20 wt % of thelubricating oil composition.
 8. The lubricating oil of claim 1, whereinthe sulfonate detergent is present in 0.01 wt % to 10 wt % of thelubricating oil composition.
 9. The lubricating oil composition of claim1, further comprising: an antioxidant, an ashless dispersant, ananti-wear agent, a detergent, a rust inhibitor, a dehazing agent, ademulsifying agent, a friction modifier, a metal deactivating agent, apour point depressant, a viscosity modifier, an antifoaming agent, aco-solvent, a package compatibilizer, a corrosion-inhibitor, a dye, oran extreme pressure agent.
 10. The lubricating oil composition of claim1, wherein the sulfonate detergent includes an internal olefinsulfonate, an alkyl ether sulfonate, an alcohol ether sulfonate, alinear alkyl aryl sulfonate, or an alkane sulfonate.
 11. A method ofimproving oxidation stability of a lubricating oil, the methodcomprising: supplying to an engine a lubricating oil compositioncomprising: a base oil; a primary antioxidant comprising alkylateddiphenylamines having an alkyl group derived from propylene tetramer;and a sulfonate detergent.
 12. The method of claim 11, wherein thelubricating oil composition further comprises: a secondary antioxidantcomprising a dithiocarbamate, a hindered phenol, or a molybdenumsuccinimide.
 13. The method of claim 11, at least 50% of the alkylateddiphenylamines have a carbon number between 10 and
 15. 14. The method ofclaim 11, wherein the sulfonate detergent is a petroleum-based detergentor a synthetic detergent.
 15. The method of claim 11, wherein theprimary antioxidant is present in 0.01 wt % to 20 wt % of thelubricating oil composition.
 16. The method of claim 11, wherein thesulfonate detergent is overbased.
 17. The method of claim 11, whereinthe secondary antioxidant is present in 0.01 wt % to 20 wt % of thelubricating oil composition.
 18. The method of claim 11, wherein thesulfonate detergent is present in 0.01 wt % to 10 wt % of thelubricating oil composition.
 19. The method of claim 11, wherein thelubricating oil composition further comprises: an antioxidant, anashless dispersant, an anti-wear agent, a detergent, a rust inhibitor, adehazing agent, a demulsifying agent, a friction modifier, a metaldeactivating agent, a pour point depressant, a viscosity modifier, anantifoaming agent, a co-solvent, a package compatibilizer, acorrosion-inhibitor, a dye, or an extreme pressure agent.
 20. The methodof claim 11, wherein the sulfonate detergent includes an internal olefinsulfonate, an alkyl ether sulfonate, an alcohol ether sulfonate, alinear alkyl aryl sulfonate, or an alkane sulfonate.